Macroporous NiO with

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Scalable Synthesis of Three-Dimensional MesoMacroporous NiO with Uniform Ultra-Large Randomly Packed Mesopores and High Catalytic Activity for Soot Oxidation Albert A. Voskanyan, and Kwong-Yu Chan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00064 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Nano Materials

Scalable Synthesis of Three-Dimensional MesoMacroporous NiO with Uniform Ultra-Large Randomly Packed Mesopores and High Catalytic Activity for Soot Oxidation Albert A. Voskanyan* and Kwong Yu Chan* The Department of Chemistry, The University of Hong Kong, Pokfulam, Hong Kong KEYWORDS: Nickel oxide, heterogeneous catalysis, soot oxidation, meso-macroporous structure, oxygen vacancies, colloidal solution combustion synthesis, nanocombustion, hard-sphere colloids

ABSTRACT Functional nanomaterials with uniform ultra-large mesoporosity possess great potential for advanced technological applications. However, their cost-effective synthesis at a large-scale still remains a major challenge. In this work, a three-dimensional (3D) hierarchical meso/macroporous NiO catalyst with uniform ultra-large randomly packed 22 nm mesopores has been synthesized for the first time by a facile, economical and highly scalable colloidal solution combustion synthesis (CSCS) and applied for catalytic soot oxidation. The structural parameters of the catalyst (specific surface area, pore volume, and pore size) can be readily tuned by the amount of colloids added and NiO with a high surface area (221.6 m2/g), a large pore volume (0.39 mL/g), and a small particle size (3-5 nm) has been produced. The addition of silica colloids confines exothermic reaction in a small nanovoids between closely arranged 22 1

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nm hard-sphere colloids leading to the combustion at a nanoscale level, which we denote as nanocombustion. Furthermore, the large surface area, uniform ultra-large mesoporosity, high crystallinity, and ultrasmall particle size of the material can be achieved at the same time, whereas post-synthesis prolonged high-temperature calcination for the crystallization usually leads to the collapse of the mesostructure in other template-assisted methods. These can be achieved since the formation and crystallization of nuclei occur almost simultaneously through the heat generated from the exothermic reaction between the reactants in the CSCS. The CSCS synthesized NiO shows excellent activity for soot oxidation resulting from the combination of interconnected accessible pores, which provide sufficient interfacial contact with the catalyst, and high concentration of oxygen vacancies. Maximum catalytic soot elimination is achieved at 394 °C, significantly better than commercial 20 nm NiO nanoparticles, which only oxidize 53% of the soot at 500 °C under tight contact condition. Considering the remarkable advantages offered, CSCS method has the required features to make the commercial-scale synthesis of high-quality porous materials reality.

Introduction Nickel (II) oxide (NiO) is an abundant, environmentally benign, and inexpensive transition metal oxide. It has received considerable attention because of its superior performance in catalysis, batteries, supercapacitors, sensors and electrochromic devices.1-8 Depending on the programmed application, NiO with various nano-dimensional structures including nanotubes, nanospheres, nanosheets, nanowires and nanoflowers have been successfully synthesized.9-15 Specifically for enhanced catalytic and electrochemical performance, a crystalline structure with large uniform mesopores is highly desirable due to the significantly improved mass transfer and diffusion rates of guest species inside and outside of the pores.16,17 However, to the best of our knowledge, there is no literature report on the successful synthesis of nanocrystalline NiO with uniform ultra-large mesopores (20-50 nm) so far.

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Generally, mesoporous oxides with uniform pore structures are synthesized using soft and hard templating methods.18-20 However, the pores generated through conventional templates like surfactants, polymer microspheres or mesoporous silicas are either too small or too large. Crystalline materials with uniform mesopores in the range of 20-50 nm are difficult to synthesize. The oxides replicated from low molecular weight surfactants or hard templates such as SBA-15 and KIT-6 have a pore size smaller than 12 nm. For example, Jiao et al fabricated crystalline NiO with uniform 11 nm pore by using KIT-6 as a hard template.21 By selecting a mesoporous SiO2 template with different pore sizes, Lai et al obtained series of NiO with pore sizes between 3-12 nm.22 On the other hand, polymeric microspheres in colloidal crystal templating method produce only macroporous materials (pore size >50 nm).23,24 To obtain oxides with uniform ultra-large mesopores tailor-made large size amphiphilic block copolymers have recently been employed in the soft templating method.25-27 However, this approach has been only successful to a few oxides yet (SiO2, TiO2 and Al2O3). Furthermore, the scale-up of the synthesis is limited due to the commercial unavailability of the suitable large size block copolymer templates, the laborious multistep synthetic method has low yield, employs expensive precursors, and has high production cost. Besides, copolymers generally undergo a big volume shrinkage and usually decompose prematurely during postsynthesis crystallization leading to a smaller pore size or even to a partial collapse of the mesostructure. A scalable and economical strategy to produce high quality, crystalline mesoporous metal oxides with ultralarge mesoporosity and accessible high surface areas is still unavailable. In the present study, we report a highly scalable and economical synthesis of 3D meso-/macroporous NiO with uniform randomly packed 22 nm mesopores via a colloidal solution combustion synthesis (CSCS). CSCS is a facile bottom-up synthesis method to mass produce crystalline nanostructures with controlled composition and tunable pore structure using hard-sphere colloids in combustion synthesis.28 Combustion synthesis utilizes internal chemical energy of reactants that releases in the form of the heat converting precursors into the products. In contrast to the other energy intensive wet chemical methods in which continuous heat supply and long reaction times are necessary for crystal nucleation and growth, a 3



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mild exothermic reaction between homogeneously mixed precursors in CSCS leads to a product formation within seconds without additional energy consumption. The combustion is mild because the silica colloids are chemically inert and they act as heat sinks absorbing generated heat from the nitrateglycine reaction. Therefore, the maximum combustion generated temperature can be readily controlled by the amount of inert colloids in the mixture. Heating is required only to initiate the ignition, to overcome the reaction activation barrier, after which the reaction becomes self-accelerating without additional external energy consumption. High-temperature released from the combustion reaction satisfies the energy requirements for crystalline oxide formation. At the same time, rapid cooling limits the crystal growth so that only small but highly crystalline NiO particles are formed. CSCS synthesized NiO shows high catalytic activity for diesel soot oxidation achieving maximum soot elimination at 394 °C under tight contact mode.

Results and Discussion The overall synthetic procedure of 3D hierarchical meso/macroporous NiO is illustrated in Scheme 1. First, a certain amount of Ni(NO3)2·6H2O used as an oxidant as well as the NiO precursor and glycine (NH2CH2COOH) used as a fuel and complexing agent were dissolved in the water. Then 22 nm spherical colloidal silica containing solution was added (Scheme 1A).

Scheme 1. Schematic illustration of NiO formation with controlled mesoporosity via CSCS method: (A) colloidal precursor solution, (B) gel formed after water evaporation, (C) combustion of nitrate-glycine precursor mixture nanoconfined between hard-sphere colloids, (D) NiO/SiO2 nanocomposite with embedded 22 nm silica colloids, (E) porous NiO with uniform 22 nm mesopores after alkaline etching. 4


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The obtained homogeneous solution was heated on a hot plate at 300 oC for 10 min in air. Following water evaporation, the volume fraction of colloids increases and they assemble to minimize the interfacial energy forming a Ni(NO3)2-NH2CH2COOH-SiO2 gel (Scheme 1B), which then ignites with heat and gas generation (Scheme 1C). The ignition takes place once oxygen radicals, which are formed from the nitrate decomposition, react with glycine molecules leading to a thermal runaway. Certainly, combustion reaction mechanism is extremely complex composing a long sequence of radical involving steps. A differential scanning calorimetry (DSC) analysis result for the gel with the optimized molar ratio of components is shown in Figure 1. It can be clearly seen that the gel ignites exothermically at around 240 o

C with 701,3 J/g heat generation. Also, the ignition can be visualized in the inset of Figure 1. After

ultrafast cooling to room temperature, a NiO/SiO2 nanocomposite containing uniformly embedded silica colloids is obtained (Scheme 1D). Finally, alkaline etching of the composite results in a highly crystalline NiO nanostructure with the desired porosity (Scheme 1E). The video and experimental details of NiO fabrication via the CSCS method are given in the Experimental Section of the Supporting Information (ESI).

Figure 1. The DSC result for optimized Ni(NO3)2-NH2CH2COOH-SiO2 gel (inset: ignition of the gel).

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For comparison, commercial NiO nanoparticles with a 20 nm average particle size (Figure S1, ESI) and NiO synthesized without colloids via conventional solution combustion synthesis were also used. Table 1 shows the volume of colloidal SiO2 added to 5 ml of NH2CH2COOH-Ni(NO3)2 precursor solution. For convenience, the resulting samples are denoted as NiO-X (where X = 0, 1, 2, 3, and 4 which stands for 0, 0.2, 0.4, 0.8 and 1 mL of SiO2 colloids added). Figure 2A-D shows typical scanning electron microscopy (SEM) images of the CSCS synthesized NiO samples.

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Figure 2. SEM images of different NiO samples (A) NiO-0, (B) NiO -2, (C) NiO-3, and (D) NiO-4. TEM (E) and HRTEM (F) images of NiO-4 sample. Noticeably, NiO-0 synthesized without silica colloids has a wide range of irregular macropores (Figure 2A). With the addition of colloids, the formation of 22 nm large mesopores can be clearly observed and 7



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the number of pores increases along with the amount of SiO2 colloids added (Figure 2B-D). Unexpectedly, NiO-2 has larger average pore size (28 nm) than other CSCS synthesized nickel oxides (Table 1). We think that it can be due to the chemical etching which may cause pore expansion in the process of colloidal silica removal. When the amount of silica is 1 ml, NiO is highly mesoporous with uniform randomly-packed spherical 22 nm pores (Figure 2D). Hard-sphere colloids are experimentally observed to always crystallize into the random hexagonal close-packed (rHCP) structure.29 The auxiliary macropores with diameters of 50-100 nm are resulted from the gas evolution during combustion. These macropores become more regular and smaller in size with increasing concentration of colloids (Figure 2A-D). The addition of silica confines reaction in small nanovoids between hard-sphere colloids, particularly when they are in high concentration, leading to the combustion at a nanoscale level, which we denote as nanocombustion. In other words, the macroreaction zone without colloids is divided into much smaller nanoreaction zones with the addition of colloids. As a result, the amount of gas generated in every individual domain will be small and will have difficulty to escape, resulting in small and relatively uniform macropores. On the other way, if there are no colloids added large amount of gas is produced at one place after rapid gas evolution because of depressurization leading to large and non-uniform macropores throughout the sample. Analogy can be drawn with bubbles of CO2 that appear when the carbonated drink is open. If the bottle is opened slowly, it takes a long time for dissolved gas to escape due to liquids surface tension. However, if the bottle is shaken or if the liquid is poured quickly into a glass, small bubbles merge together forming larger bubbles, which can escape very quickly, creating a fizz. The formation of well-defined mesopores which are composed of small nanoparticles can also be seen from the transmission electron microscopy (TEM) images in Figure 2E and 2F. The formation of such small crystallites is the result of extremely short reaction time combined with an ultrafast cooling. For example, by using a transient heat conduction method the calculated time required for the heat dissipation from 500 oC to 30 oC is around 0.17 ms for a spherical and isolated 5 nm NiO particle exposed in air at 20 oC (the calculation is given in ESI). In other words, the time during which the generated oxide 8


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nuclei are exposed to the high temperature is very short, which is enough for crystallization, but not enough for extended crystal growth. Furthermore, hard silica colloids function as both heat sinks dissipating the heat and as physical barriers preventing oxide crystals from further growth. The high-resolution TEM (HRTEM) image of a single mesopore (Figure 2F) clearly shows a circular circumference of the wall, which is composed of 3-5 nm crystalline NiO nanoparticles exposed in different orientations. The oxide particles are linked through the grain boundaries (GBs), which are known to be highly active for catalytic oxidation reactions due to the enhanced oxygen diffusion in GBs compared with the bulk of the crystals.30,31 From the HRTEM image, the calculated spacing between lattice-resolved fringes is 0.24 nm, corresponding to (111) planes of cubic NiO. The selected area electron diffraction (SAED) in the Figure S2 displays a ring pattern further confirming a well-crystallized oxide formation. The measured d spacing values are in good agreement with the fcc NiO structure. The porous features of oxides were further analyzed by N2 sorption measurements.

Table 1. Textural parameters of NiO catalysts.

NiO-1

NiO-2

NiO-3

NiO-4

NiO-comm.

V(SiO2) colloids 0 added (mL)

0.2

0.4

0.8

1

N/A

SBET (m2 g-1)

93.4

154.6

182.2

221.6

76

BJH Pore volume 0.09 (ml g-1)

0.2

0.28

0.34

0.39

0.13

Average pore size 10.4 (nm)

22.3

28.4

22.3

22.3

6

XRD particle size 20.1 (nm)

9.1

7.6

6.3

5.2

13

Sample

NiO-0

34.6

The isotherms of NiO-2, NiO-3 and NiO-4 samples demonstrated a type-IV isotherm with a steep capillary condensation step on adsorption branch in P/Po range of 0.85-0.95, indicating the formation of 9



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ultra-large mesopores (Figure S3, ESI). The results of BET surface area, BJH pore volume and average pore size are listed in Table 1. NiO-4 has the largest surface area (221.6 m2 g-1), and the largest pore volume (0.39 ml g-1) among the other NiO samples with narrow pore size distribution centred at 22 nm (Figure 3A), which is consistent with the TEM results. Additionally, a small tail in 50-100 nm region indicates the presence of macropores. As shown in Figure S4, the total pore volume and specific surface area of the oxides increase dramatically with the addition of silica, confirming successful colloidal templating.

Figure 3. (A) BJH pore size distribution plots, (B) XRD patterns, (C) XPS of Ni 2p core level, and (D) XPS of O 1s core level of different NiO catalysts. The X-ray diffraction (XRD) was used to analyze the crystalline structure and the purity of the prepared oxides (Figure 3B). All diffraction peaks can be indexed to the crystalline rocksalt fcc NiO phase (Fm3m, JCPDS 00-047-1049). From Figure 3B, the peak width increases as the amount of silica 10


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added increases, indicating a decrease in the crystalline size of NiO. The average particle size was calculated via the Scherrer equation using (200) peak and results are listed in Table 1. As expected crystalline size decreases with the SiO2 amount added with the NiO-4 sample having the smallest size of 5.2 nm. The calculated particles size agrees well with the HRTEM results shown in Figure 2F. To further evaluate the surface chemical composition, NiO catalysts were studied by X-ray photoelectron spectroscopy (XPS) (Figure 3C and D). The Ni 2p XP spectra of the NiO-4 is compared together with NiO synthesized by conventional SCS (NiO-0) and with the commercial 20 nm NiO nanoparticles. High-resolution scans of Ni 2p3/2 and 2p1/2 peaks are centered at 854 and 871 eV, respectively, with a spin-energy separation of 17 eV which corresponds well with the literature data for Ni2+.32 These main peaks also have shake up satellites located at 862 and 879 eV. Both peaks comprise of two components which are due to Ni2+ (t2g6 eg2) and Ni3+ (t2g6 eg1) species. The relative concentration of nickel species with different oxidation states has been obtained after deconvolution of peaks and results are listed in Table S2. The concentration of Ni3+ was calculated considering the area of both Ni3+ peaks. As can been seen from the results in Table S2, commercial NiO, NiO-0 and NiO-4 have almost the same concentration of Ni3+ on the surface. The high concentration of Ni3+ does not affect the lattice structure as indicated in XRD. The O 1s region is also analyzed and shown in Figure 3D. The binding energy of the oxygen atom varies depending on whether it is bonded as lattice oxygen, adsorbed oxygen or molecular water.33 The O 1s core level curves of NiO exhibit two distinct peaks centered at 529, 531 and shoulder peak at 532 eV. The peak at 529 eV is assigned to lattice O2-, whereas the high-binding energy peaks at 531 and 532 eV are attributed to adsorbed oxygen species, and bounded water molecules, respectively. The relative concentrations of these species over the total surface oxygen amount are determined and also listed in Table S2. Importantly, NiO-4 has more adsorbed oxygen (58%) than Ni-O-Ni on the surface, which can be attributed to the high concentration of oxygen vacancies.34,35 NiO-4 has 17% more oxygen vacancies than NiO-0 and NiO-comm, although they have almost the same concentration of surface Ni3+ species. In fact, the increase in the concentration of cations with lower valence state to higher valence 11



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increases the concentration of oxygen vacancies. The high concentration of oxygen vacancies in NiO-4 may be due to the oxygen vacancy segregation to grain boundaries. We employed as-synthesized NiO samples for catalytic soot oxidation in a pure oxygen atmosphere as a model reaction. Mass spectrometry confirmed the absence of impurities in oxygen and helium inlets. In future studies, it can be meaningful to study catalytic activity in other strong oxidants as NOx, which can be present in vehicle exhausts. Catalytic soot oxidation primarily depends on the catalyst composition, structure and contact mode with the soot. Since the typical soot particle size ranging between 10-50 nm (Figure S5), catalysts with uniform ultra-large mesopores (20-50 nm) and auxiliary macropores (>50 nm) are highly desirable for efficient soot oxidation due to the enhanced catalyst-soot contact number and minimized mass and heat transfers.36-38 It should be noted that distinguishing soot and catalyst particles using standard SEM or TEM characterization techniques is very challenging due to the insufficient contrast between them. Even if the soot particles may not deeply penetrate into the ultra-large mesopores, however they will still be in a good contact with the pore openings or partially enter into them enhancing soot/catalyst interfacial contact. Additionally, the high concentration of surface oxygen vacancies is also of paramount importance, providing active sites for gas phase oxygen activation.39-41 The results for soot oxidation catalyzed by various NiO catalysts are illustrated in Figure 4A. Notably, NiO-4 catalyst shows the highest activity, reaching light-off temperature T50% at 362 oC (T50% is the temperature at which 50% of soot is combusted) and maximum conversion is achieved at 394 o

C corresponding to complete removal of the soot in the upstream reactor, while NiO-0 only achieves

82% conversion at 465 oC. CO2 is the main oxidation product (SCO2 = 99.8%) and only traces of CO were observed. The temperature values T10, T50, and T90 of the soot oxidation under tight contact conditions are listed in Table 2. The results shown in Figure 4A and Table 2 depends on the contact mode, flow rate of the gas and the heating rate. At a higher oxygen flow rate and isothermal condition, the soot can be completely burned out at a lower temperature (burnout temperature). We have kept the gas flow and

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heating rate to be consistent but have not explored effects of varying these parameters on the burnout temperature.

Figure 4. (A) Soot oxidation results for different NiO catalysts tightly mixed with soot particles with 50 ml min-1 gas feed of 4% O2 and balance He, and a heating rate of 10 oC/min (catalyst:soot ratio 5:1 wt./wt.) and (B) O2-TPD profiles of the catalysts. CO2 concentration in y axis is relative to concentration at peak temperature. Peak concentration temperature represents the temperature required to ensure complete soot oxidation.

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Table 2. The catalytic soot oxidation activity of the NiO catalysts under tight contact conditions with 50 ml min-1 gas feed of 4% O2 and balance He, and a heating rate of 10 oC/min (catalyst:soot ratio 5:1 wt./wt.). Catalysts

NiO-0

NiO-2

NiO-3

NiO-4

NiO-comm. Soot

T10 (oC)

342

300

320

300

363

508

T50 (oC)

420

377

386

364

457

550

T90 (oC)

-

424

424

387

-

567

The catalytic activity decreases in the sequence of NiO-4 > NiO-3 ≈ NiO-2 > NiO-0 > NiO-comm. Among these five nickel oxide catalysts, the commercial 20 nm NiO nanoparticles demonstrate the lowest performance, oxidizing only 53% of soot at 500 oC. The pure soot burns non-catalytically between 550570 oC. Although the commercial NiO has around two times larger surface area than NiO-0, nevertheless it shows a lower activity. The better activity of NiO-0 over commercial NiO is probably due to its macroporous structure (Figure 2A), which may allow deep penetration of soot particles and thus ensures more interfacial contact points with the catalyst. It is generally accepted that although catalysts with three-dimensional ordered macroporous (3DOM) structures possess relatively low surface areas, they show high catalytic activity owing to the enhanced soot-catalyst contact areas.42-45 It has been demonstrated that the soot oxidation activity of Ag/CeO2 is determined by the soot-ceria contact number rather than the catalyst surface area.46 The key for the efficient soot oxidation is the transfer of reactive oxygen species (ROS) such as superoxide radicals (·O2-) and hydroxyl radicals (·OH-) from the oxide surface to the soot through contact points or by a spillover mechanism.46,47 To further investigate the presence of ROS on the catalysts surface, the O2-TPD experiments were performed and the results are shown in Figure 4B. The O2 desorption at low-temperature range of 150-350 oC is attributed to the ROS (α-O2) adsorbed on the oxygen vacancies, whereas desorption peak at 400-600 oC is ascribed to the 14


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release of lattice oxygen (β-O2).48 The small peaks below 150 oC are related to the desorption of weakly physisorbed molecular O2. The concentration of ROS on NiO-4 catalyst is significantly higher compared with both commercial NiO and NiO-0, which is in a good agreement with O 1s XPS results. Although desorption maximum of α-O2 species for commercial NiO is slightly shifted to a lower temperature compared with NiO-0, however, it shows much lower activity. This suggests that the tight contact of the catalyst with soot particles and also the presence of sufficient amount of surface ROS have a critical role in the efficient soot elimination. Furthermore, the high concentration of desorbed β-O2 from NiO-4 catalyst indicates the high mobility of lattice oxygen, which can also participate in the oxidation process contributing to overall catalytic activity. To assure the importance of tight contact we also performed soot oxidation under loose contact condition using the NiO-4 catalyst. The results in Figure S6 demonstrate that there is no catalytic reaction under loose contact mode and therefore tight contact is necessary for the low-temperature soot oxidation. Besides, to test the durability of the NiO-4, three successive oxidation cycles were performed under tight contact, and the results are shown in Figure 5.

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Figure 5. Catalytic stability tests of the CSCS synthesized 3D hierarchical meso-macroporous NiO-4 catalyst with three cycles for soot oxidation under tight contact conditions and a heating rate of 10 oC/min (catalyst:soot ratio 10:1 wt./wt.). The CO2 concentration profiles exhibit a small shift towards higher temperature within three oxidative cycles, demonstrating a good stability. In addition to catalytic durability, the NiO-4 has also an excellent mechanical stability. There is no collapse of the porous structure after grinding the powder in the agate mortar as indicated by the TEM image (Figure S7). Moreover, even after the soot combustion, there is still some porosity retention as shown in Figure S8. Without the use of colloids, there is little control and uniformity of pore sizes which are mainly in the macropore domain as shown in Figure 2A and 3A for NiO-0. The CSCS method gives an excellent control of mesopore diameter with relatively small amount of macropores formed. Therefore, CSCS provides a platform for systematic investigation of pore size on the catalytic activity by employing hardsphere colloids with the diameter between 10-200 nm. On the basis of the obtained results it can be concluded that the high catalytic activity of CSCS synthesized NiO-4 results from the combination of redox properties, highly open hierarchical meso/macroporous structure, and high concentration of oxygen vacancies. Due to the high concentration of oxygen vacancies, more catalytically active ROS are present on the oxide surface, while an open hierarchical porous structure with uniform large pores provides more interfacial contact points with the catalyst. The catalytic activity of NiO-4 for the soot oxidation compares favorably with other transition metal-containing catalysts reported in the literature as summarized in Table S3. To demonstrate the large-scale production of NiO-4, 100 g of the catalyst (Figure S9) has been synthesized within 24 h via CSCS with a low production cost and details are given in the Experimental Section. In addition to a large-scale synthesis capability, other advantages provided by the CSCS method are exceptional compared to other well-established template-assisted methods. Specifically, the large surface area, uniform ultralarge mesoporosity, high crystallinity, and ultrasmall particle size of the 16


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catalyst can be achieved at the same time, whereas post-synthesis prolonged high-temperature calcination for the crystallization usually leads to the collapse of the mesostructure in other template-assisted methods. These can be achieved since the formation and crystallization of nuclei occur almost simultaneously through the heat generated from the exothermic reaction between the reactants in the CSCS. Besides, the possibility of controlling the interaction forces between the colloids gives a fascinating opportunity to tune the structure of materials produced. For example, by carefully manipulating the surface charge of the colloids by adding inert salts, changing the pH of the precursor solution, valence of the counterion or by adding other components (e.g. short chain polymers) may produce materials with a different pore arrangement and structure, which will be explored in the future work. We believe that the CSCS method can be readily extended to other 3D hierarchical mesomacroporous materials with desired properties for various advanced applications, such as heterogeneous catalysis, sorbents, sensors, disordered photonics or energy conversion and storage.

Conclusions In summary, using a simple, highly scalable, and cost-effective CSCS method a crystalline 3D NiO with uniform randomly packed 22 nm mesopores was successfully synthesized for the first time. It was demonstrated that the structure of the NiO (such as mesopore diameter, specific surface area, and pore volume) can be readily tuned by the amount of silica colloids added. The addition of silica confines reaction in a small nanodomains between close-packed 22 nm hard-sphere colloids resulting in combustion at a nanoscale level. Furthermore, hard silica colloids function as both heat sinks dissipating the heat and as physical barriers preventing oxide crystals from further growth. The ultrafast cooling after combustion inhibits extended crystal growth forming a pore circumference composed of 3-5 nm NiO particles. Due to the synergy of unique 3D open interconnected meso-/macroporous structure and high concentration of oxygen vacancies, the as-prepared NiO-4 exhibit excellent catalytic activity for soot oxidation achieving soot elimination at 394 oC, while 20 nm NiO nanoparticles only oxidize 53% of the 17



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soot at 500 oC under tight contact condition. The CSCS satisfies most criteria to be a powerful synthesis method for mass producing structure-controlled porous nanomaterials and opens up new horizons in a field

of

materials

science.

ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge. Calculation of heat dissipation time via a transient heat conduction method, TEM image of a commercial NiO sample, SAED pattern of NiO-4 sample, N2 adsorption-desorption isotherms of NiO samples, Surface area and pore volume of different NiO samples prepared by CSCS versus amount of silica colloids added, TEM image of soot particles, Soot oxidation result for NiO-4 catalysts under loose

contact condition, TEM image of the NiO-4 after grinding in the agate mortar for 10 min, TEM image of the NiO-4 catalyst after soot oxidation, Image of NiO-4 powder produced via CSCS, Catalytic activity results of different transition metal containing catalysts for the soot oxidation, Synthesis of NiO-4 catalyst via CSCS method (combustion starts at 1 min 10 s). (PDF)

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected] (A. A. V.) *E-mail: [email protected] (K.Y. C.)

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT 18


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The authors acknowledge financial support from The University of Hong Kong (HKU) Seed Funding for Basic Research and University Development Fund (UDF) for Initiative for Clean Energy and Environment (ICEE). Research was partly supported by an Innovation Technology Fund GHP/048/14 and Research Grants Council Award GRF 17300014. The authors sincerely thank Frankie Chan of the Electron Microscopy Unit at HKU for assistance during TEM characterization.

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TOC


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Macroporous NiO with

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